 
  

 






<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html>

<!-- Mirrored from www.javapractices.com/topic/TopicAction.do?Id=12 by HTTrack Website Copier/3.x [XR&CO'2010], Sun, 12 Jun 2011 17:28:08 GMT -->
<!-- Added by HTTrack --><meta http-equiv="content-type" content="text/html;charset=UTF-8"><!-- /Added by HTTrack -->
<head>
 <title>
  Java Practices -> Copy constructors
 </title>
 <link rel="stylesheet" type="text/css" href="../stylesheet8.css" media="all">
 
 <link rel="shortcut icon" href='../images/favicon.ico' type="image/vnd.microsoft.icon">
 <meta name="description" content="Concise presentations of java programming practices, tasks, and conventions, amply illustrated with syntax highlighted code examples.">
 
 <meta name='keywords' content='clone,constructor,java,java programming,java practices,java idiom,java style,java design patterns,java coding conventions,'>
 
 
</head>
 
<body>


<div class='menu-bar'>
 
  <a href='../home/HomeAction.html' title='Table of Contents'>Home</a> |
  <a href='../vote/VoteSummaryAction-2.html' title='View Poll Results'>Poll</a> |
   
  <A href='../feedback/FeedbackAction451f-2.html?Operation=Show' title='Send Your Feedback'>Wiki</a> |
  <b><a href='../source/SourceAction-2.html' title='Grab Source Code'>Source Code</a></b><IMG class='no-margin' SRC="../images/goldstar.gif" ALT=""> |

  <a href='http://www.web4j.com/Java_Web_Application_Framework_Overview.jsp?From=1' title='Free Download - Java Web Application Framework'><b>WEB4J</b></a> |
  
  <a href='http://www.date4j.net/' title='Replacement for java.util.Date'><b>DATE4J</b></a> |

   <a href='../references/ReferencesAction-2.html' title='References'>Links</a>
   
  <form action='http://www.javapractices.com/search/SearchAction.do' method='get' class='search-form'>
   <input type='text' name='SearchTerms' value="" size=12 maxlength=50 class='search'>
   <input type='submit' value="Search">
  </form>
 
</div>

<P>



  

 






<p class="display-messages">

 

 

</p>


<div class="main-layout">
 
   

 




<div class='page-title'>Copy constructors</div>

<div class='main-body'>
 
<br>Copy constructors :
<ul>
<li>
provide an attractive alternative to the rather pathological <tt><a href="TopicActioned8c-2.html">clone</a></tt>
method</li>

<li>
are easily implemented</li>

<li>
simply extract the argument's data, and forward to a regular constructor</li>

<li>
are unnecessary for <a href='TopicActiond838-2.html'>immutable objects</a></li>
</ul>
<b>Example</b>
<br>
<PRE>

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Galaxy {

  <span class='keyword'>public</span> Galaxy (<span class='keyword'>double</span> aMass, String aName) {
     fMass = aMass;
     fName = aName;
  }

  <span class='comment'>/**
  * Copy constructor.
  */</span>
  <span class='keyword'>public</span> Galaxy(Galaxy aGalaxy) {
    <span class='keyword'>this</span>(aGalaxy.getMass(), aGalaxy.getName());
    <span class='comment'>//no defensive copies are created here, since 
</span>    <span class='comment'>//there are no mutable object fields (String is immutable)
</span>  }

  <span class='comment'>/**
  * Alternative style for a copy constructor, using a static newInstance
  * method.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> Galaxy newInstance(Galaxy aGalaxy) {
    <span class='keyword'>return</span> <span class='keyword'>new</span> Galaxy(aGalaxy.getMass(), aGalaxy.getName());
  }

  <span class='keyword'>public</span> <span class='keyword'>double</span> getMass() {
    <span class='keyword'>return</span> fMass;
  }

  <span class='comment'>/**
  * This is the only method which changes the state of a Galaxy
  * object. If this method were removed, then a copy constructor
  * would not be provided either, since immutable objects do not
  * need a copy constructor.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>void</span> setMass( <span class='keyword'>double</span> aMass ){
    fMass = aMass;
  }

  <span class='keyword'>public</span> String getName() {
    <span class='keyword'>return</span> fName;
  }

  <span class='comment'>// PRIVATE /////
</span>  <span class='keyword'>private</span> <span class='keyword'>double</span> fMass;
  <span class='keyword'>private</span> <span class='keyword'>final</span> String fName;

  <span class='comment'>/**
  * Test harness.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main (String... aArguments){
    Galaxy m101 = <span class='keyword'>new</span> Galaxy(<span class='literal'>15.0</span>, <span class='literal'>"M101"</span>);

    Galaxy m101CopyOne = <span class='keyword'>new</span> Galaxy(m101);
    m101CopyOne.setMass(<span class='literal'>25.0</span>);
    System.out.println(<span class='literal'>"M101 mass: "</span> + m101.getMass());
    System.out.println(<span class='literal'>"M101Copy mass: "</span> + m101CopyOne.getMass());

    Galaxy m101CopyTwo = Galaxy.newInstance(m101);
    m101CopyTwo.setMass(<span class='literal'>35.0</span>);
    System.out.println(<span class='literal'>"M101 mass: "</span> + m101.getMass());
    System.out.println(<span class='literal'>"M101CopyTwo mass: "</span> + m101CopyTwo.getMass());
  }
} 
</PRE>
<br>
<br>Example run of this class :
<p><tt>>java -cp . Galaxy</tt>
<br><tt>M101 mass: 15.0</tt>
<br><tt>M101Copy mass: 25.0</tt>
<br><tt>M101 mass: 15.0</tt>
<br><tt>M101CopyTwo mass: 35.0</tt>
<br>
<br>

</div>




<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicAction79f1-2.html?Id=15'>Defensive copying</a> <br>
 
  
  <a href='TopicActiond838-2.html?Id=29'>Immutable objects</a> <br>
 
  
  <a href='TopicActioned8c-2.html?Id=71'>Avoid clone</a> <br>
 
</div>


<div class='topic-section'>Would you use this technique?</div>
<div class='main-body'>
  
  <form action="http://www.javapractices.com/vote/AddVoteAction.do" method='post'>
    Yes<input type='radio' name='Choice' value='Y' >
    &nbsp;&nbsp;No<input type='radio' name='Choice' value='N'>
    &nbsp;&nbsp;Undecided<input type='radio' name='Choice' value="?" >
    &nbsp;&nbsp;<input type=submit value="Vote" >
    <input type='hidden' name='Operation' value='Apply'>
    <input type='hidden' name='TopicId' value='12'>
  </form>
</div>

<div style='height:10.0em;'></div>

 
 
</div>

  

 





<div align='center' class='legalese'>  
&copy; 2011 Hirondelle Systems |
<a href='../source/SourceAction-2.html'><b>Source Code</b></a><IMG class='no-margin' SRC="../images/goldstar.gif" ALT=""> |
<a href="mailto:webmaster@javapractices.com">Contact</a> |
<a href="http://creativecommons.org/licenses/by-nc-sa/1.0/">License</a> |
<a href='../apps/cjp.rss'>RSS</a>
<!-- ukey="2AC36CD2" -->
<!-- ckey="16DF3D87" -->
<br>

 Individual code snippets can be used under this <a href='../LICENSE.txt'>BSD license</a> - Last updated on June 6, 2010.<br>
 Over 150,000 unique IPs last month - <span title='Java Practices 2.6.5, Mon May 16 00:00:00 EDT 2011'>Built with</span> <a href='http://www.web4j.com/'>WEB4J</a>.<br>
 - In Memoriam : Bill Dirani -
</div>

<script src="../../www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-2633428-1";
urchinTracker();
</script>



</body>

<!-- Mirrored from www.javapractices.com/topic/TopicAction.do?Id=12 by HTTrack Website Copier/3.x [XR&CO'2010], Sun, 12 Jun 2011 17:28:08 GMT -->
<!-- Added by HTTrack --><meta http-equiv="content-type" content="text/html;charset=UTF-8"><!-- /Added by HTTrack -->
</html>
